Making colored plastic products is often a three step process, with each step taking place at a different facility. In a first process, a plastic resin, such as polyethylene, polypropylene, polystyrene, or other resin, is synthesized and pelletized for sales at a production facility. In a second step, at another facility the pelletized resins are compounded with an additive, such as a colorant masterbatch, and formed into colored pellets. As used herein, a masterbatch is a material blended with a high concentration, e.g., as much as 70% or more, by weight, of a pigment or other additive. In a final process, the colored plastic pellets are formed into a final product, such as a pipe, sheet, film, bottle, and the like. In some circumstances, the colorant may be blended with the resin at the facility producing the plastic product.
Due to the risk of contamination, colorant is generally not used at the production facility and more generally is not used in large extruders that are used to produce uncolored products in addition to colored products. Thus, color compounding facilities have traditionally been entirely separate facilities with associated intermediate storage and handling facilities. Constructing and maintaining a separate compounding facility can be very expensive, and may often cost as much as $US20-30 million, or more. Thus, lower cost systems for forming colored products would be beneficial.
In some applications, sequential extruders have been used to provide different mixing environments to solve problems with mixing resins that have different characteristics. For example, a small side-feed extruder can be used to melt additives, such as masterbatches, for feeding to a larger blending extruder. In another example, a first extruder may provide an aggressive blending environment to melt a resin quickly. The melted resin may then be fed to a second extruder that provides a gentler environment for a longer period of time to blend in more sensitive resins. An application using this type of arrangement is a bimodal blending line, configured to blend a high molecular weight polyethylene with a low molecular weight polyethylene. For example, in U.S. Pat. No. 7,815,360 a material processing plant with two screw-type extruding machines is disclosed, wherein a first extruding machine discharges a melt through a linkage to a lower lying extruder. Other systems having multiple sequential or cascading extruders have been described, for example, in European Patent Application Publication No. 1 005 411 B1, U.S. Pat. No. 3,261,056, and German Patent Publication No. 2 304 088 A.
A number of systems have been developed that use a side extruder to melt a feed, or form a compound, and then force it into a main extruder for blending with a polymer system. For example, U.S. Patent Application Publication No. 2009/0057621 discloses electrically conductive compositions and methods for forming these compositions. In the method described, a molten masterbatch is mixed with a first polymer in a first extruder. The first polymer has a melt viscosity that is lower than the melt viscosity of the molten masterbatch, and the melt viscosity of the molten masterbatch is reduced to form a reduced viscosity molten masterbatch. The reduced viscosity molten masterbatch is mixed with a second polymer in a second extruder to form the electrically conductive composition.
U.S. Patent Publication No. 2008/0093763 discloses multi-color fiber-plastic composites and systems and methods for forming the composites. In the application, multiple compounding extruders are used to form melts that are forced into a primary extruder for blending with a base polymer.
In addition to these systems, any number of other systems can use side feed extruders to feed melted materials into a main extruder, including commercially available systems from Coperion GMBH, of Stuttgart, Germany, (under the ZS-B trade name), among others. However, all of the prior systems are configured to combine melted materials for blending and forming a single plastic product. Thus, the prior systems are configured to send all of the melt formed in a first extruder into a second extruder.
As described above, forming colored plastics is generally performed in a multistep process, with the resins pelletized and cooled between each step. Although this may lower or eliminate the risk of cross-contamination of a natural, or uncolored, resin with a color, it subjects the resin to multiple heat histories. The subsequent heat input steps also deplete the additives used to protect the plastic resin from UV, heat and other aggressive environments so that additional additives may be required in continuing process/fabrication steps. Furthermore, the costs of building separate intermediate storage and compounding facilities can substantially increase the costs of the colored resins and products.
Thus, there remains a need for improved processes for forming colored plastics. Furthermore, there is a need for a method that would enable one to form different plastic products from a single melt stream.